Methods: Patients >18 years old who underwent primary one- or two-level TLIFs at our institution were identified and propensitymatched in a 3:1 fashion (unilateral:bilateral). Patient demographics, surgical characteristics, and radiographic outcomes, including vertebral endplate obliquity, segmental lordosis, subsidence, and fusion status, were compared between groups. Results: Of the 184 patients included, 46 received bilateral cages. Bilateral cage placement was associated with greater subsidence (1.06±1.25 mm vs. 0.59±1.16 mm, p=0.028) and enhanced restoration of segmental lordosis (5.74°±14.1° vs. −1.57°±10.9°, p=0.002) at the 1-year postoperative point, while unilateral cage placement was associated with an increased correction of endplate obliquity (−2.02°±4.42° vs. 0.24°±2.81°, p<0.001). Bilateral cage placement was significantly associated with radiographic fusion on bivariate analysis (89.1% vs. 70.3%, p=0.018) and significantly predicted radiographic fusion on multivariable regression analysis (estimate, 1.35; odds ratio, 3.87; 95% confidence interval, 1.51–12.05; p=0.010). Conclusions Bilateral interbody cage placement in TLIF procedures was associated with restoration of lumbar lordosis and increased fusion rates. However, endplate obliquity correction was significantly greater for patients who received a unilateral cage.

Methods: All patients aged >18 years who underwent primary one- to four-level ACDF at a single institution were retrospectively identified. Propensity matching was performed to compare patients’ PEEK or titanium alloy cages with structural allograft. Multivariate logistic regression analysis was performed to measure the effect of interbody spacer composition on the likelihood of pseudarthrosis development. Results: Of the 502 patients who received structural allograft and had 1-year postoperative dynamic radiographs, 96 patients were propensity matched to 32 patients who received a PEEK cage, and 162 patients were propensity matched to 54 patients who received a titanium alloy cage. Multivariate logistic regression analysis identified that PEEK cage implants (odds ratio [OR], 3.34; p =0.007) predicted pseudarthrosis development compared with structural allograft implantation. Titanium alloy cage (OR, 1.64; p =0.156) implantation was not predictive of pseudarthrosis. One-year postoperative PROMs were not significantly different between patients who received PEEK or titanium alloy cages and those who received structural allograft (all p >0.05). Conclusions Compared with structural allograft, receiving a PEEK cage increased the risk of pseudarthrosis development following ACDF, whereas receiving a titanium alloy cage had no significant effect on pseudarthrosis development. One-year postoperative patient-reported outcomes were similar between patients who received structural allograft, PEEK, and titanium alloy interbody spacers.